necessary

from The American Heritage® Dictionary of the English Language, 5th Edition.

• adjective Needed or required: synonym: indispensable.
• adjective Unavoidably determined by prior conditions or circumstances; inevitable.
• adjective Logically inevitable.
• adjective Required by obligation, compulsion, or convention.
• noun Something indispensable.

from The Century Dictionary.

• Such as must be; that cannot be otherwise.
• Such that it cannot be disregarded or omitted; indispensable; requisite; essential; needful; required: as, air is necessary to support animal life; food is necessary to nourish the body.
• In law:
• Requisite for reasonable convenience and facility or completeness in accomplishing the purpose intended: as, the land necessary for building a railroad.
• Naturally and inseparably connected in the ordinary course: as, necessary consequences.
• Acting from compulsion or the absolute determination of causes: opposed to free. See free.
• Synonyms Necessary, Essential, Requisite, Needful. The following remarks refer to the application of the words to ordinary practical affairs, not to philosophy. Necessary is so general a word that it covers all the others, and has the additional sense, which they do not have, of inevitable. Essential is an absolute word, noting that which is a part of the chief end of the action, or of every mode of bringing that end about. Requisite is less strong than essential, and needful is less strong still; yet each is strong and emphatic, applying to that which is imperatively needed. Needful generally applies to concrete, and often to temporary, things: as, knowledge of the countries visited is requisite, and even essential, to enjoyment of travel, but money is needful in order to be able to travel at all. Needful is often applied to that which must be supplied to produce or effect a perfect state or action.
• noun Anything that is necessary or indispensable; that which cannot be disregarded or omitted: as, the necessaries of life.
• noun A privy; a water-closet.

from the GNU version of the Collaborative International Dictionary of English.

• adjective Such as must be; impossible to be otherwise; not to be avoided; inevitable.
• adjective Impossible to be otherwise, or to be dispensed with, without preventing the attainment of a desired result; indispensable; requisite; essential.
• adjective Acting from necessity or compulsion; involuntary; -- opposed to free.
• noun A thing that is necessary or indispensable to some purpose; something that one can not do without; a requisite; an essential; -- used chiefly in the plural.
• noun A privy; a water-closet.
• noun (Law) Such things, in respect to infants, lunatics, and married women, as are requisite for support suitable to station.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective needed, required
• noun archaic, UK bathroom, toilet, loo

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun anything indispensable
• adjective unavoidably determined by prior circumstances
• adjective absolutely essential